Why would he refuse to fly if the MEL says it's good to go
How about because the MEL
allows the captain to accept a defect, it does not
require him/her to do so.
The MEL does not gauruntee it's safe, because it does not (can not) possibly cater for all the possible combinations of circumstances. Aircraft I've flown have usually had some legal weasil wording to that effect anyway.
So we exercise judgement, that's why they pay us the big bucks.
Most of the time the MEL provides us with the operational flexibility we need, but sometimes it's just one more slice of swiss cheese with the hole lined up.
pb